Multiple Engines???

It seems that for rockets of multiple stages with only one fuel combination,
there is an interesting engineering decision.

Consider a two stage rocket where both stages burn the same fuel
combination.

You could use 1 engine for the upper stage, and 4-ish engines of the same
design for the lower stage. The advantage is that you need only design one
engine type. The disadvantage is that with 4-ish engines on the lower stage
you probably cannot tolerate an engine failure, and clearly not a catastrophic
failure. Therefore you might lose a bit of reliability (which you might get back by spending the saved money on reliability).

Alternatively, you might use two different engine designs, a large
and a small. This reduces the total part count while increasing the
total unique part count. It probably increases cost and reliability.

Does anyone have any numbers that might help convince which is the
better path? For example, is motor design cost a large part of the
overall vehicle cost? Are most failures due to motor failures? Is a
single large motor likely to weigh less and/or have a higher ISP than
a few smaller (but still large) motors?

Basically, anyone have any good arguments for either choice?

-Thanks
-Talleyrand

P.S. Is it reasonably easy to tailor an engine to
atmosphere or vacuum operation with changes to the engine bell;
things like turbopump and cooling systems can remain the same?

"Charles Talleyrand" wrote in message
...
It seems that for rockets of multiple stages with only one fuel
combination,
there is an interesting engineering decision.

Consider a two stage rocket where both stages burn the same fuel
combination.

You could use 1 engine for the upper stage, and 4-ish engines of the same
design for the lower stage. The advantage is that you need only design
one
engine type. The disadvantage is that with 4-ish engines on the lower
stage
you probably cannot tolerate an engine failure, and clearly not a
catastrophic
failure.

Use five, one in the center, and design so that 3.5 or 4 at rated thrust
would be sufficient. Then, run 5 at 80-90%; if the center engine dies, the
others can be revved up. If an outer one dies, the opposite one can be cut
back for balance while the others rev up.
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The important point is that propellant is cheap, cheap, cheap, and loss of a launcher is as expensive as all hell.

Best reliability calls for a completely reuseable single stage launcher, with engines of such a size and number that, if at any
time one of them must be shut down, the others throttle up to compensate, and you just keep going. Use of two parallel stages is
a distant second because separating two vehicles at high speed in the atmosphere is not simple. (A two parallel stage launcher
can be designed in which separation occurs outside the atmosphere; this does make the return of the first stage a bit more
difficult.) Series staging is terrible because you take off without the second stage engines running, and thus without knowing
whether they will start and run correctly. In any case there's little reason to use multiple different engine designs in one
vehicle.

The "reusable" part means that you can get all of the design and manufacturing errors out of every flight article before it goes
into service. This also means that, in service, the chances of a catastrophic engine failure are negligible.

Best reliability calls for a completely reuseable single stage launcher

Alas, SSTO fuel fraction is prohibitive. 2STO typically uses 1/3 the
propellant for a given payload, although vehicle empty weights are higher.

separating two vehicles at high speed in the atmosphere is not simple.

Shuttle does it every mission - SRBs from ET, ET from Orbiter.

Series staging is terrible because you take off without the second stage engines running, and thus without knowing whether they will start and run correctly.

Agreed

In any case there's little reason to use multiple different engine designs in one vehicle.

Servicing a single engine type is cheaper, if all other things are equal.
4 on the booster and 1 on the Orbiter would support a single engine core,
with differing bell arrangements.

"Richard Schumacher" wrote in message ...

The important point is that propellant is cheap, cheap, cheap, and loss of a launcher is as expensive as all hell.

Best reliability calls for a completely reuseable single stage launcher, with engines of such a size and number that, if at any
time one of them must be shut down, the others throttle up to compensate, and you just keep going.

This is not obvious.

In a world where engines explode upon failure, having exactly one engine per stage is best.

In a world where there are finite development dollars, and those dollars can buy reliability, having
one TYPE of engine per vehicle is best.

It has been suggested to me in private email that the correct answer to this delima is to have one engine
FAMILY, but with multiple engine sizes per family.


The "reusable" part means that you can get all of the design and manufacturing errors out of every flight article before it goes
into service. This also means that, in service, the chances of a catastrophic engine failure are negligible.

I dunno. Seems to me that airplanes occasionaly suffer failure despite their resability, and it's not clear why
repeated testing makes catastrophic failure preferencially less likely than any other type.

Richard Schumacher wrote:

The important point is that propellant is cheap, cheap, cheap, and loss of a launcher is as expensive as all hell.

Best reliability calls for a completely reuseable single stage launcher, with engines of such a size and number that, if at any
time one of them must be shut down, the others throttle up to compensate, and you just keep going. Use of two parallel stages is
a distant second because separating two vehicles at high speed in the atmosphere is not simple. (A two parallel stage launcher
can be designed in which separation occurs outside the atmosphere; this does make the return of the first stage a bit more
difficult.) Series staging is terrible because you take off without the second stage engines running, and thus without knowing
whether they will start and run correctly.

If you don't get enough engines running to reach orbit you have to
abort. That's terrible only if a flight costs you hundreds of millions
of dollars, like the Shuttle. I was on a flight once that had to do an
"RTLS" abort because of engine problems. The worst part of it was that
we missed the end of the movie (Rocky).

In any case there's little reason to use multiple different engine
designs in one
vehicle.

The "reusable" part means that you can get all of the design and manufacturing errors out of every flight article before it goes
into service. This also means that, in service, the chances of a catastrophic engine failure are negligible.

By the way, 80 or less column posts are usually appreciated.
Format, format, format!

Charles Talleyrand wrote:
"Richard Schumacher" wrote:
The important point is that propellant is cheap,
cheap, cheap, and loss of a launcher is as expensive as all hell.

Best reliability calls for a completely reuseable single stage
launcher, with engines of such a size and number that, if at any
time one of them must be shut down, the others throttle up to
compensate, and you just keep going.

This is not obvious.

In a world where engines explode upon failure, having exactly
one engine per stage is best.

Engines actually rarely explode on failure; going back
through the history of flight failures shows almost exclusively
systems failure followed by shutdown, or accidental shutdown,
without any uncontained failure. It's not unknown but is a lot
rarer than 'graceful' shutdowns.

Cost and complexity constraints as well as reliability
analysis do argue for single engines per stage on expendables,
and five or more on reusables with abort-to-orbit (fewer if
abort-to-ground is ok).

In a world where there are finite development dollars,
and those dollars can buy reliability, having
one TYPE of engine per vehicle is best.

That does have its limits. It's great on SSTO and Stage
and a Half, and some TSTO concepts. It sucks on three or
more stage vehicles where the GLOW of the first stage may be
a hundred or more times the GLOW of the last stage...

It has been suggested to me in private email that the correct
answer to this delima is to have one engine
FAMILY, but with multiple engine sizes per family.

Hard to do that; engines don't scale very well in terms of keeping
similar design and construction. Similar operating concept and
specifications? Sure. But the parts won't be descended or
related very closely.

One reasonable exception is truncated versus long nozzles...
*that* isn't such a big change. You can keep the exact same
pumps, combustion chamber, injector, etc.

The answers to some of these questions vary significantly when
you look at serious RLV operability and seriously far out BDB
designs, as optimizations start to pull in unexpected ways.


-george william herbert

Am 20 Nov 2003 20:38:05 -0800 schrieb "George William Herbert":

In a world where engines explode upon failure, having exactly
one engine per stage is best.

Engines actually rarely explode on failure; going back
through the history of flight failures shows almost exclusively
systems failure followed by shutdown, or accidental shutdown,
without any uncontained failure. It's not unknown but is a lot
rarer than 'graceful' shutdowns.

Ok, then search for "hard start" instead - that's the euphemism used
for engine explosion when it occurs on ignition. You will maybe
surprised to find a significant number of them...

cu, ZiLi aka HKZL (Heinrich Zinndorf-Linker)
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In article ,
Charles Talleyrand wrote:
...The disadvantage is that with 4-ish engines on the lower stage
you probably cannot tolerate an engine failure...

You can come close. The Saturn V, with five engines on the first stage,
had only a couple of brief time windows in which it was unable to survive
a single engine failure. Mind you, in many cases it would not be able to
complete its mission, but at least it would remain under control and give
you time to think about whether there was any way to salvage the mission.

You really need 6-8 engines to be able to fly the mission with an engine
out at any time. (The Saturn I twice carried on successfully after losing
one of eight first-stage engines -- once a deliberate test, once a real
failure.)

...For example, is motor design cost a large part of the
overall vehicle cost?

It's a significant part of the vehicle cost, although how much depends on
details. It also tends to take longer than vehicle development, so if you
start them at the same time, the vehicle will be waiting on the engines.
(Both the F-1 and the J-2 were started before the Saturn V. The SSME was
a major cause of shuttle development delays.)

Are most failures due to motor failures?

"Most" is an overstatement, but at least some assessments have found that
it's the single largest cause. (The number of data points is not large
and how you classify some of them is rather subjective.)

How many engines you can afford to *lose* is an important number. If the
answer is "none", you want as few engines as possible, ideally 1. But
with most reasonable sets of assumptions, overall vehicle reliability
improves substantially once you start being able to lose one and carry on,
because the fault tolerance outweighs the larger number of engines.

(One of the assumptions you have to make, of course, is what fraction of
engine failures are catastrophic, since one of *those* may cause damage
you can't survive even if the loss of thrust would be okay. However,
except in cases of gross abuse -- e.g., failing to shut the engine down
when the tanks run dry, allowing the high-speed turbopumps to suck air --
failures of fully-developed liquid-fuel engines are almost always benign.)

Is a
single large motor likely to weigh less and/or have a higher ISP than
a few smaller (but still large) motors?

Other things being equal, it's not likely to make much difference.
Neither thrust/weight nor Isp depends much on size, except at the extreme
low end, where performance tends to deteriorate some (at least for
orthodox design approaches).

Mind you, larger engines are notoriously more prone to combustion
instability. And their inflexible geometry can be hard to fit into
a vehicle. But those are secondary issues, "mere engineering". :-)

Basically, anyone have any good arguments for either choice?

Depends on your priorities. A demand for low cost pushes toward
simplicity. A demand for high reliability pushes toward fault tolerance.

P.S. Is it reasonably easy to tailor an engine to
atmosphere or vacuum operation with changes to the engine bell;
things like turbopump and cooling systems can remain the same?

There are some compromises involved. (In particular, if you want the
cooling system to stay the same, you probably can't regeneratively cool
the changeable part of the nozzle.) So it won't be quite as good as
engines built specifically for particular conditions. But it has been
done; the penalties are not huge.
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In article ,
David Shannon wrote:
Best reliability calls for a completely reuseable single stage launcher

Alas, SSTO fuel fraction is prohibitive.

Not necessarily. When people have been pushed hard to try to build
expendable stages with that sort of fuel fraction, they have generally
succeeded. And with 1960s technology, too, in some cases.

Reusability is the uncertain part, but that's true of TSTO systems too.

2STO typically uses 1/3 the
propellant for a given payload, although vehicle empty weights are higher.

However, since propellant costs are negligible, and empty mass and
complexity are the expensive parts...

separating two vehicles at high speed in the atmosphere is not simple.

Shuttle does it every mission - SRBs from ET, ET from Orbiter.

Note the words "in the atmosphere". The ET separation occurs in vacuum.

The SRB separation may look simple but it isn't; NASA spent a lot of time
and money making sure it would work.

Servicing a single engine type is cheaper, if all other things are equal.
4 on the booster and 1 on the Orbiter would support a single engine core,
with differing bell arrangements.

In fact, NASA planned roughly that for the original two-reusable-stage
shuttle.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |